The present invention relates to a hydraulic actuator for controlling relative movement between first and second structures between which the actuator is mounted and more particularly to a hydraulic actuator having some having integral suspension.
Elongate structures, for example booms which are commonly used in various applications, are typically subjected to large bending forces, requiring some flexibility in order to prevent failure of the structure. In the agricultural industry for instance, booms are used for various application including spraying crops and the like. When displacing a boom of this type across a field, the boom may be subjected to varying bending forces due to uneven terrain, cornering and acceleration of the vehicle supporting the boom. Suspension of the boom thus must accommodate these varying bending forces to prevent failure of the boom structure.
Often elongate structures, including booms and the like, however must be collapsible for transport due to the irregular shape of the structures. Collapsing such a structure involves pivotally mounting the structure on a base frame and providing an actuator for displacing the structure between deployed and undeployed positions. Common actuators, such as hydraulic actuators, however are very rigid in both fully extended and retracted positions of the actuator and thus additional suspension is required to accommodate the flexibility needed of the structure.
In place of additional suspension when using a hydraulic actuator, the use of limit switches are known to prevent the actuator from being fully extended or retracted in normal operation such that some flexibility of the actuator is permitted when an accumulator or pressure relief is provided. The use of limit switches however results in a costly and complex design which is far less reliable than the hydraulic actuator itself.
According to one aspect of the present invention there is provided a hydraulic actuator for controlling relative movement between first and second structures between which the actuator is mounted, the actuator comprising:
a cylinder having a mounting end for coupling to one of the structures and an open end opposite the mounting end;
a cushion piston slidably mounted within the cylinder;
a stop member arranged to restrict displacement of the cushion piston away from the mounting end of the cylinder beyond a prescribed spacing from the mounting end of the cylinder in a static position of the cushion piston;
a biasing mechanism urging the cushion piston away from the mounting end of the cylinder towards the static position adjacent the stop member, the cushion piston being movable against the biasing mechanism;
a rod slidably mounted through the open end of the cylinder with the open end of the cylinder being sealed with respect to the rod, the rod having a piston end slidably mounted in sealing engagement within the cylinder between a retracted position adjacent the cushion piston and an extended position adjacent the open end of the cylinder and having a mounting end for coupling to the other structure; and
hydraulic fluid supply lines coupled to both a retraction port adjacent the open end of cylinder in communication with a first side of the piston end of the rod to retract the rod into the retracted position and an extension port adjacent the mounting end of the cylinder in communication with a second side of the piston end of the rod to extend the rod into the extended position;
the rod being movable with the cushion piston towards the mounting end of the cylinder beyond the retracted position when subjected to a force which exceeds a prescribed holding force of the biasing mechanism.
The cushion piston provides some flexibility to the actuator to accommodate the varying forces between a pair of relatively movable structures between which the actuator is mounted. The cushion piston requires no additional electrical switches or controllers and can be part of the hydraulic circuitry of the actuator so as to provide a simple, low cost actuator with integral suspension which is effective and reliable unlike any prior known device.
In a preferred embodiment, the cushion piston is sealed with respect to the cylinder and the biasing mechanism comprises a cushion chamber under hydraulic pressure within the cylinder between the cushion piston and the mounting end of the cylinder.
Pressure of hydraulic fluid within the cushion chamber and at the retraction port are preferably substantially equal in the retracted position. Accordingly, a cross sectional area of the cushion piston in communication with the cushion chamber is preferably greater than a cross sectional area of the first side of the piston end of the rod in communication with the retraction port.
There may be provided a pressure relief mechanism in communication with the cushion chamber and the retraction port so as to permit the rod to be deflected from the retracted position in either direction. In one embodiment, the pressure relief mechanism includes a hydraulic fluid accumulator in communication with each of the cushion chamber and the retraction port, permitting hydraulic fluid to be received therein when a prescribed holding force of the accumulator is exceeded.
When the hydraulic fluid lines are coupled in communication with the cushion chamber, a hydraulic switch is preferably arranged to selectively couple the hydraulic fluid lines to an outlet and a return reservoir of a hydraulic fluid pump.
The cushion chamber and the retraction port may be arranged to communicate with one another at a junction between the hydraulic switch and the cylinder. A flow restrictor is preferably coupled in series with the hydraulic fluid lines between the junction and the retraction port to ensure the cushion chamber is pressurised first.
There may be provided a check valve coupled in communication with a respective one of the hydraulic fluid supply lines of each of the retraction and extension ports, the check valve of each port being operable when the respective hydraulic fluid supply line of the other port is pressurised.
The piston end of the rod is preferably engaged upon the cushion piston in the retracted position of the rod. When the stop member is mounted between the piston end of the rod and the cushion piston, the stop member preferably spans only partway into the cylinder to permit communication of the piston end of the rod and the cushion piston across the stop member.
There may be provided a projection engaged between the cushion piston and the piston end of the rod to displace the cushion piston from the stop member before the piston end of the rod engages the stop member when the rod is deflected from the retracted position towards the mounting end of the cylinder.
The stop member is preferably fixed in relation to the cylinder.
The hydraulic actuator may be provided in combination with a boom structure pivotally supported on a base frame structure for movement relative to the base frame structure, the mounting end of the cylinder and the mounting end of the rod each being coupled to a respective one of the structures. Preferably the mounting end of the cylinder is mounted on the base frame structure and the mounting end of the rod is mounted on the boom structure.
The hydraulic actuator may also be provided in combination with an agricultural sprayer comprising:
a base frame structure having a tank supported thereon for rolling movement along the ground; and
a boom structure pivotally supported on the base frame structure for pivotal movement relative to the base frame structure, the boom structure having spraying lines and nozzles supported thereon in communication with the tank on the base frame;
the mounting end of the cylinder and the mounting end of the rod each being coupled to a respective one of the structures.
The hydraulic actuator in this instance preferably controls pivotal movement of the boom structure relative to the base frame structure about an upright axis.
The hydraulic actuator may control pivotal movement of the boom structure relative to the base frame structure between a folded position in which the boom structure extends generally in a direction of travel of the sprayer and a field position in which the boom structure lies transversely to the direction of travel. The rod is preferably oriented so as to be in the retracted position of the rod when the boom structure is in the field position.